Use the logging module directly instead of home-grown code.

This has the advantage of overall less custom code; as well as support
for per-module configuration. This would enable a potential solution for
ultrabug#1479, since in the future
it can allow per-module configuration of log levels.

I expect this to mainly help module creators, allowing them to enable
logging for only their module, while disabling all other messages. It
also is easier to use, since the `logging` module's interface is
generally simpler than `self.py3`.

Here, I've made the decision to keep the message format as close as
possible to the existing log messages.

py3status/core.py

@@ -29,6 +29,12 @@
     "info": logging.INFO,
 }
 
+LOGGING_LEVELS = {
+        "error": logging.ERROR,
+        "warning": logging.WARNING,
+        "info": logging.INFO,
+}
+
 DBUS_LEVELS = {"error": "critical", "warning": "normal", "info": "low"}
 
 CONFIG_SPECIAL_SECTIONS = [